1. Industrial Field of Application
The present invention relates to a semiconductor device using crystalline semiconductor, and to a method for fabricating the same.
2. Discussion of Prior Art
Thin film transistors (referred to simply hereinafter as xe2x80x9cTFTsxe2x80x9d) are well known as devices utilizing thin film semiconductors. The TFTs are fabricated by forming a thin film semiconductor on a substrate and processing the thin film semiconductor thereafter. The TFTs are widely used in various types of integrated circuits, and are particularly noticed in the field of switching elements that are provided to each of the pixels of active matrix liquid crystal display devices as well as in driver elements of the peripheral circuits thereof.
Amorphous silicon films can be utilized most readily as the thin film semiconductors for TFTS. However, an amorphous silicon film has a problem that the electrical characteristics thereof are inferior. This problem can be circumvented by using a thin film of crystalline silicon. Crystalline silicon film is also denoted as, for example, polycrystalline silicon, polysilicon and microcrystalline silicon. A thin film of crystalline silicon can be prepared by first forming a thin film of amorphous silicon, and then crystallizing it by heat treatment.
The heat treatment for the crystallization of the amorphous silicon film requires heating the film at a temperature of 600xc2x0 C. or higher for a duration of 10 hours or longer. Such a heat treatment has a problem that a glass substrate cannot be used. For instance, a Corning 7059 glass commonly used for the substrate of-an active matrix liquid crystal display device has a glass distortion point of 593xc2x0 C., and is therefore not suitable for large area substrates that are subjected to heating at a temperature of 600xc2x0 C. or higher.
According to the study of the present inventors, it is found that the crystallization of an amorphous silicon film can be effected by heating the film at 550xc2x0 C. for a duration of about 4 hours. This can be accomplished by first introducing a trace amount of nickel or palladium, or other elements such as lead, into the surface of the amorphous silicon film.
The elements above (catalyst elements capable of accelerating the crystallization of an amorphous silicon film) can be introduced into the surface of the amorphous silicon film by plasma treatment or vapor deposition, or by ion implantation. The plasma treatment is a method comprising adding the catalyst elements onto the amorphous silicon film by generating a plasma of an atmosphere such as gaseous nitrogen or gaseous hydrogen in a plasma CVD apparatus of a parallel plate type or of a positive column type, while using a material containing catalyst elements as an electrode.
However, the presence of the catalyst elements in a large quantity in the semiconductor is not preferred, because the use of such semiconductors greatly impairs the reliability and the electric stability of the device in which the semiconductor is used. That is, the elements such as nickel which accelerate the crystallization (catalyst elements) are necessary in the crystallization of the amorphous silicon film, but are preferably not incorporated in the crystallized silicon. These objects can be accomplished by selecting an element which tends to be inactive in crystalline silicon as the catalyst element, and by incorporating the catalyst element at a minimized amount for the crystallization of the film. Accordingly, the quantity of the catalyst element to be incorporated in the film must be controlled with high precision.
Also, in case of using nickel as the catalyst element, a crystalline silicon film was fabricated from an amorphous silicon film by adding nickel by plasma treatment, and the crystallization process and the like was studied in detail to obtain the following findings as a result:
(1) In case of incorporating nickel by plasma treatment into an amorphous silicon film, nickel is found to intrude into the film to a considerable depth of the amorphous silicon film before subjecting the film to heat treatment.
(2) The initial nucleation occurs from the surface from which nickel is incorporated.
(3) When a nickel layer is deposited on the amorphous silicon film by vapor deposition, the crystallization of an amorphous silicon film occurs in the same manner as in the case of effecting plasma treatment.
It can be concluded from the above findings that not all of nickel atoms incorporated by plasma treatment into the amorphous silicon film function effectively, and that, more importantly, only a trace amount of nickel need to be incorporated in the vicinity of the surface of the amorphous silicon film. Assumably, a point (or a plane) at which silicon is brought into contact with nickel contributes to the low temperature crystallization of amorphous silicon. Conclusively, nickel atoms are preferably dispersed as finely as possible to accelerate the crystallization reaction. In other words, xe2x80x9cnickel atoms need to be introduced in the vicinity of the surface of amorphous silicon film at a minimum concentration necessary for the low temperature crystallization of the amorphous silicon filmxe2x80x9d.
A trace amount of nickel, i.e., a catalyst element capable of accelerating the crystallization of the amorphous silicon, can be incorporated in the vicinity of the surface of the amorphous silicon film by, for example, vapor deposition. However, vapor deposition is disadvantageous concerning the controllability of the film, and is therefore not suitable for controlling precisely the amount of the catalyst element that is incorporated in the amorphous silicon film.
In particular, the crystals can be grown in parallel with the plane of catalyst element the silicon film from the region onto which the solution is applied to the region onto which the solution is not applied. It is also confirmed that this region of crystal growth contains the catalyst element at a low concentration and that it is extremely useful to utilize such a crystalline silicon film as an active layer region for a semiconductor device. However, there remains a problem how to selectively introduce the catalyst elements.
An object of the present invention is to provide a method for fabricating a thin film semiconductor of crystalline silicon, characterized in that it satisfies the following requirements:
(1) The catalyst element is introduced at a controlled and at a minimum possible quantity;
(2) The catalyst element is introduced into selected portions; and
(3) The process yields high productivity.
The present invention uses the following means to accomplish the object above. Specifically, a mask-patterned amorphous silicon film is crystallized by bringing it into contact with either a pure catalyst element which accelerates the crystallization of the amorphous silicon film or a compound containing the catalyst element, while applying heat treatment thereto.
More specifically, a solution containing the catalyst element is applied to the surface of an amorphous silicon film having a desired pattern formed thereon using a resist. In this maimer, the catalyst element is introduced into the surface of the amorphous silicon film.